We're using socket.io for mobile-server communications. Since we can't force-upgrade users' devices, if we want to upgrade to version 1 (non-back-compatible), we have to handle both versions on the server for a while.
What are the options?
My current favourite is to wrap both the old version and the new version in a multiplexer. It detects the version of the incoming request based on headers and query parameters and thereby knows which functions to invoke.
Another (shittier) option is to wrap the new version in a module that can translate the old version of the protocol into the new version (and back again) when necessary. This suffers from a serious drawback. It would be time-consuming and uncertain work to ensure I've properly determined and handled all the tiny differences. Some differences might take some serious massaging.
(In case you're curious or it's helpful to know, we're doing this in Go.)